Life table study of Sitotroga cerealella on different cereals and its implications on the performance of the egg parasitoid (Trichogramma chilonis) under laboratory conditions

Sitotroga cerealella is one of the major pests of cereals in the field and storage conditions throughout the world. The main objective was to study the life tables of S. cerealella on wheat, maize and barley and its implications on percent parasitism of Trichogramma chilonis. S. cerealella is reared under lab conditions as its eggs are utilized for rearing T. chilonis. Fresh eggs of S. cerealella were collected and after hatching the neonate larvae of S. cerealella were transferred onto each host plant species for obtaining first (F1) generation (G). Seventy eggs were used for each host and each egg was used as a replicate. Daily observations were made for recording the life-table parameters of the S. cerealella. The data showed that the developmental time of S. cerealella eggs and pupae was maximum (5.68 and 7.75 days) when reared on wheat, while the maximum larval duration (19.77 days) of S. cerealella was recorded on barley. The maximum fecundity (290.30 ± 22.47 eggs/female) was recorded on maize, while minimum fecundity per female was recorded on barley (159.30 eggs/ female). The S. cerealella reared on maize had significantly higher values of finite rate of increase (λ), intrinsic rate of increase (r), and net reproductive rate (Ro) (0.14 ± 0.04 day− 1, 1.16 ± 0.05 day− 1, and 136.85 ± 20.25 eggs/ female) respectively. The mean generation time (T) (35.18 ± 0.61 days) was higher on wheat. Likewise, the gross reproductive rate (GRR) and the age-stage specific reproductive values (vxj) of newly oviposited eggs of S. cerealella were recorded higher (136.85 ± 20.25; 1.160 offspring) on maize. The data regarding the efficacy of T. chilonis for different parameters were recorded higher on maize i.e., percent parasitism (89.00 ± 2.30%), percent adult emergence (81.60 ± 1.20%), adult longevity (3.80 ± 0.10 days) and total adult longevity (9.90 ± 0.20 days) as compared to wheat and barley. Our findings revealed that S. cerealella can be best reared on maize under laboratory conditions as it prefers this host as compared to wheat and barley. Therefore, assigning the most susceptible and favorite host (maize) would help us to improve T. chilonis mass production under laboratory conditions.


Results
The results showed that the food source affects the population parameters of S. cerealella ( Table 1). The eggs developmental time (days) of S. cerealella was significantly lower when reared on maize and barely (5.45 and 5.43 days, F = 278.57, DFs = 2, 207), respectively as compared to the developmental time of S. cerealella eggs reared on wheat (5.68 days) (P = 0.0167). The total larval duration (days) of S. cerealella when reared on maize was recorded significantly lower (P = 0.0079, 17.97 days) as compared to S. cerealella reared on barley (19.77 days), while non-significant difference was observed when reared on maize and wheat (P = 0.4826, (F = 228.02, DFs = 2, 207). The data regarding the pupal duration (days) of S. cerealella show that significantly lower pupal duration of S. cerealella was recorded when reared on maize (6.57 ± 0.12 days) (P = 0.00, (F = 1142.59, DFs = 2, 195) as compared to wheat and barley, while non-significant difference was recorded in the pupal duration of S. cerealella reared on wheat (7.75 ± 0.14 days) and barley (7.42 ± 0.11 days) (P = 0.076). In the same way, significantly lower pre-adult survival rate of S. cerealella was recorded when reared on maize (30.69 ± 0.36 days) (P = 0.001, F = 0.002, DFs = 2, 207), while the greatest pre-adult survival rate of S. cerealella was recorded on barley (33.28 ± 0.33 days), followed by wheat (32.87 ± 0.39 days). The total developmental time of S. cerealella adults was significantly extended on maize (11.22 ± 0.30 days, F = 64.09, DFs = 2, 207) and barley (10.27 ± 0.32 days) as compared to wheat (10.90 ± 0.40 days) (P = 0.016). The total pre oviposition period (TPOP) of S. cerealella reared individually on barley and wheat was not significantly different (P = 0.5615). A significantly higher number of oviposition days of S. cerealella were observed when reared on maize (6.96 ± 0.360 days) (P = 0.00, F = 431.06, DFs = 2, 96), while no significant difference was observed in the oviposition days of S. cerealella reared on wheat and barley (P = 0.300). In the same way, significantly maximum number of eggs (fecundity/ female) of S. cerealella was recorded when     www.nature.com/scientificreports/ from eggs of S. cerealella reared on maize followed by barley (72.00 ± 1.53%) while the minimum mean percent adult emergence of T. chilonis was recorded (72.00 ± 1.53%) on eggs reared on wheat. Maximum adult longevity (3.80 ± 0.12 days) and total longevity (9.96 ± 0.20 days) of T. chilonis were recorded on S. cerealella eggs reared on maize while significantly lower adult longevity (3.066 ± 0.12 days) and total longevity (7.90 ± 0.32 days) of T. chilonis were recorded on S. cerealella eggs reared on wheat.

Discussion
Life table studies of insects reared on different hosts can reflect the suitability of the different hosts for that specific pest species 26 . Several researchers have reported that the life table parameters of the stored grain pests could be affected by the physiochemical properties (morphology and hardness of the grain, nutritional value and availability of the food) of the stored products 27 . In the current study, the lifetable and population parameters of Angoumois grain moth, S. cerealella were studied individually on wheat, barley and maize. The greatest fecundity and oviposition period was recorded when S. cerealella was reared on maize as compared to barley and wheat. The eggs and larval developmental time (days) of S. cerealella was recorded lower on maize as compared to wheat and barley. Similarly, the developmental time of pupae (days) was observed lower on maize as compared to wheat and barley. Also, minimum male and female total longevity (days) was recorded when S. cerealella was reared on maize as compared to wheat and barley respectively. In the same way the total Pre-Ovi-Position period (TPOP), doubling time and pre adult survival rate (%) was recorded lower on maize and wheat as compared to barley. In previous studies, the population parameters of S. cerealella reared on the different hosts were reported by other researchers. A study conducted by Chi and Su reported that rapid growth of insects on a given diet suggests high   www.nature.com/scientificreports/ host suitability 28 . Further the loss of weight, emergence of adult and gain in adult weight was observed by stored insect pests on susceptible varieties as compared to resistant varieties 24 . The highest intrinsic rate of increase (r) per day for S. cerealella was recorded on maize and the mean generation time (T) of S. cerealella was recorded on wheat. Similarly, the highest finite-rate of increase (λ) per day, net reproductive-rate (R 0 ), age specific fecundity (f x ), age specific maternity (l x m x ), life expectancy of both adults, age stage specific reproductive values (v xj ) and Gross reproductive rate (GRR ) of S. cerealella were recorded on maize. The greatest age-stage specific life expectancy (e xj ) of newly oviposited eggs of S. cerealella was recorded on barley. Also, the net reproductive rate (R 0 ) and gross reproductive rate (GRR ) metrics only measure the reproductive potential of diets, not their total fitness. Therefore, care should be taken when using these two metrics (R 0 and GRR ) to assess if a diet is suitable for S. cerealella. Similar results were reported for S. cerealella when it was reared on maize 29 , wheat 20 and rice 30 30 . Similarly, the age specific life expectancy of a newly oviposited egg of S. cerealella was also recorded higher on barley as compared to wheat and maize. These three hosts were further assessed for their proximate compositions (Supplementary file "Table S1"). The proximate results showed higher moisture (12.5%), crude protein (13.00%), ash content (2.20%) and crude fat (3.60%) in maize as compared to wheat and barley, while no significant different were recorded in % carbohydrate content in all the three types of grains. These results further confirm that these grain's constituents have a direct effect on the biology of the S. cerealella 31 . Grains with high moisture, crude protein and ash contents are positively correlated with the progeny emerged and susceptibility index 31,32 . Similarly, grains with high crude fats have positive impact on the mean fecundity data of S. cerealella 10 . These findings support the current research finding where maximum mean number eggs laid of S. cerealella was recorded when reared on maize grains.
The projection of S. cerealella population reared on wheat, barley and maize reflects the usefulness of age-stage two-sex life tables for forecasting the population under favorable environmental conditions. According to the simulation results the population growth was the quickest on maize and it was followed by wheat and slowest on barley. The S. cerealella population on maize reached 15,679.72 adults, while on wheat it reached 1,577.56 adults and on barley reached 1,004.87 adults after 60 days. Predicting and anticipating the population growth of insect pests are critical in formulating the correct timing schedule for a pest management programme 34 . A computerbased program using life table data is therefore a very important tool in pest management and decision making 35 .
The mean parasitism, mean adult emergence, longevity of adult and total adult longevity of T. chilonis were recorded highest on S. cerealella eggs reared on maize. A research study conducted by Akter et al., on the lifetables of T. chilonis feeding on S. cerealella reported that T. chilonis parasitism ranged from 61 to 95%, with adult emergence 51 to 96.30%. They further reported adult longevity ranged from 4.2 to 7.4 days 33 . Current findings are further on par with the work of Nadeem et al., who recorded the greatest adult's emergence (98%) of T. chilonis from host eggs 36 . According to evolutionary models, more females will oviposit in large hosts than in small hosts 37,38 . A similar study conducted by Khan et al., also reported the same parameters of T. chilonis reared on S. cerealella egg under free choice method and observed percent parasitism, percent adult emergence, adult longevity and total longevity of adults as 94.0 ± 0.33, 94.0 ± 0.49, 9.6 ± 0.16 and 3.6 ± 0.27) days respectively 27 . Similarly, Charnov et al. also stated that sex-ratio manipulation may be an adaptation in response to the differing impact of host size on the fitness of female versus male wasps 39 . Our findings are further in line with these studies in that large size eggs from maize produced proportionately more female wasps than smaller size eggs from wheat and barley, which produced the lowest proportion of females but still produced more females than males.
Based on present study, S. cerealella completed their life cycle on all tested grains but maize was the best food source among the tested grains. The simulation data results showed that the growth of the S. cerealella population was faster on maize than it was on wheat and barley. Also, the results show that different food sources used for rearing of host insect significantly affected the percent parasitism, adult emergence, and adult longevity of the T. chilonis. The current study provides valuable results since understanding the life-tables study of S. cerealella on various hosts will be useful in mass production of biocontrol agents. Also, utilizing a susceptible host (maize) could help researchers and farmers in improving mass production of biocontrol agents. Further study on the www.nature.com/scientificreports/ demographic parameters of S. cerealella can help to understand the population dynamics and to develop more effective management programs for this pest.

Conclusions
The study results showed that S. cerealella completed its life cycle on all the grains however maize seems to be a better food source for mass production of this pest compared to wheat and barley. The simulation data results showed that the growth of the S. cerealella was faster on maize as compared to wheat and barley. Understanding the life table studies of S. cerealella on different hosts is useful in designing a comprehensive scheme for an IPM program of S. cerealella. Additionally, T. chilonis parasitism rate, adult emergence, and adult longevity were affected by the food sources of host insects. Therefore, assigning the most susceptible and favorite host (maize) would help us to improve T. chilonis mass production. These findings will provide a concrete basis for further studies in the real environment for both S. cerealella and T. chilonis.

Materials and methods
The study was conducted at the Department of Plant Protection, The University of Agriculture Peshawar during 2020. The insecticide-free seeds of wheat (variety Saleem 2000), barley (variety AAJ) and maize (cultivar Azam) were purchased from the local market in Peshawar. The seeds were disinfected by sterilization at 121 °C for 30 min in an autoclave. After sterilization, the seeds were air dried and then were used for the experiments. Population projection. The population potential and growth of S. cerealella reared on wheat, maize, and barley were utilized for population and growth projection of S. cerealella by the Chart i.e., TIMING-MS program 40,41 . For the initial population of S. cerealella on maize, wheat, and barley, 10 eggs in each host were used to project the population growth of S. cerealella for 60 days duration.

Rearing of the
Rearing of Trichogramma chilonis. When enough host eggs were available, rearing of T. chilonis was initiated. Sticky gum was spread uniformly on a piece of hard paper (15 × 10 cm). Then fresh eggs (300) of the S. cerealella (F1 generation) reared on each host were sprinkled on each sticky card pieces. These egg cards were then exposed to UV radiation for one hour to kill the larvae present inside eggs. After the radiation treatment, these eggs cards were transferred to a glass jar containing adults of T. chilonis in the ratio of 6:1 (female: male) for 3 h. After 3 h, the exposed egg cards were kept in separate glass jars under laboratory conditions at 25 ± 2 °C, 65 ± 5% R.H. with 08:16 h (L: D) cycle. T. chilonis adults upon emergence were fed on 10% honey drops solution. The emergence of T. chilonis adults was programmed i.e., emergence of adults from parasitized eggs which have been parasitized by keeping them at low temperatures (5 °C) during the pupal stage.

Efficacy of Trichogramma chilonis against Sitotroga cerealella eggs under laboratory conditions.
To study the efficacy of T. chilonis against S. cerealella reared on wheat, barley and maize, fresh eggs of S. cerealella (100) of F2 generation from each host were collected separately from already established cultures. Those eggs were then scattered uniformly on sticky cards (3 × 2 cm). A total of five cards each having 20 eggs/ card were used for each treatment. These egg cards were then placed separately in the glass jar containing freshly mated female adult of T. chilonis for parasitization for 24 h. The exposed eggs were then kept in separate glass jar under laboratory conditions at 25 ± 2 °C, 65 ± 5% R.H. with 08:16 h (L: D) cycle until they turned black indicating development of the parasitoid inside. Upon turning black, the cards containing eggs were collected from the jars and were examined for percent parasitism. Percent Parasitism was calculated using formula 42  Statistical analysis. Data on eggs hatching, larval and pupal durations, adult emergence, adult longevity, preoviposition period and oviposition period were analyzed utilizing age stage, two sex life-table theory 26,28,40,43,44 . The Intrinsic rate of increase (r), the finite rate of increase (λ) and net reproductive rate (R 0 ) were calculated with the help of a TWOSEX-MS Chart program. Bootstraps methods (10000 resampling) were used for the estimation of standard errors and comparison of population parameters of S. cerealella reared on wheat, barley and maize hosts. The data regarding the efficacy studies of T. chilonis were subjected to one way analysis of variances. The data on mean parasitism, mean adult emergence, adult longevity and mean (total) adult longevity were analyzed using Statistical package (Statistix 8.1) means were compared using LSD test at 5% level of probability.
Ethical declaration. All the experiments were reviewed and approved by the Department of Plant Protection, The University of Agriculture, Peshawar ethical committee as per university set guidelines and were in accordance with the Pakistan Agricultural Pesticide Act. 1997.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.